It has been proposed to provide one or more air-bags in a motor vehicle, each air-bag being provided with a gas supply adapted to be activated in the event that an accident should occur, the gas inflating the bag to form a cushion to protect the driver of the vehicle or an occupant of the vehicle.
It has been proposed before, see DE-A-4002662, to use compressed air initially contained in a bottle to inflate an air-bag. The bottle is opened by igniting a pyrotechnic charge which serves to separate, from the bottle, a lid which is initially formed integrally with the rest of the bottle, there being a relatively thin break line between the lid and the bottle. An arrangement of this type does not have any problems relating to the initial sealing of the bottle, but a lot of energy is expended in removing the lid from the bottle.
It has also been proposed to use compressed air contained in a bottle which is sealed by a locking element pressed against an outlet port for the bottle. DE-A-2121389 discloses an arrangement of this type. However, the closure element must be pressed very firmly against the outlet port to ensure a totally air-tight seal, and a lot of energy is required to remove the locking element from the sealed position.
One disadvantage in using a system in which compressed air is released from a bottle is that when compressed air is released from a bottle or other receptacle which contains the compressed air, the air tends to cool. It is a well-known law of physics that the volume of a fixed quantity of a gas, such as air, is substantially proportional to its temperature. Thus the cool air does not efficiently inflate the air-bag.
It has also been proposed to utilise a pyrotechnic source of gas to inflate an air-bag. Such a source may also provide problems in that gas from a pyrotechnic source is frequently extremely hot and can thus damage the interior of the air-bag unless special precautions are taken. Also, many of the gases generated by a pyrotechnic source may be toxic or poisonous, especially if present in a significant concentration.
Although, as explained above, the use of a pyrotechnic source is associated with problems, it is to be understood that problems are also associated with the use of compressed air initially contained in a bottle or other receptacle to inflate an air-bag. Typically the pressure of such compressed air can be 400 bar (=40 MPa). When a compressed air-bottle of this type is opened, there is a very high initial flow, that flow itself being at high pressure. This leads to a very high "volume-flow". As, at least initially, the volume of the bag is very small, the part of the bag that is actually inflated by this flow of air, travels at a very high speed. Consequently, a dangerously high momentum is imparted to the bag and to the gas associated with the bag. The part of the bag that has this momentum is the part of the bag that is closest to the person that it is desired to protect, and in some instances, the part of the bag having the high momentum can actually hit or impinge upon the person to be protected by the bag, which can cause unnecessary injuries. It is to be appreciated that as the volume of the bag increases, the pressure within the compressed air-bottle falls, which means that there is then a much lower volume-flow, and since this volume-flow is being introduced into an already considerable volume, the gas-bag only inflates at a relatively slow rate during the final stages of its inflation.
It is most desirable to have an almost opposite characteristic to that described above. The ideal situation would involve a relatively small initial flow of gas, thus causing the bag to begin inflation in a slow and gradual manner, without any part of the bag having a high speed, with the flow-rate increasing when the interior of the bag defines a significant volume, so that the air-bag is fully inflated within an acceptable period of time. It is presently felt that the maximum acceptable period of time for the inflation of an air-bag is 60 milliseconds.
Whilst a pyrotechnic or gas generator provides an almost constant flow of gas over a period of time, thus being a little closer to the desirable characteristic described above, it is to be appreciated that even a pyrotechnical gas generator (which is associated with various problems described above) is not ideal.